External Charger for a Medical Implantable Device Using Field Inducing Coils to Improve Coupling

ABSTRACT

By incorporating magnetic field-inducing position determination coils (PDCs) in an external charger, it is possible to determine the position of an implantable device by actively inducing magnetic fields using the PDCs and sensing the reflected magnetic field from the implant. In one embodiment, the PDCs are driven by an AC power source with a frequency equal to the charging coil. In another embodiment, the PDCs are driven by an AC power source at a frequency different from that of the charging coil. By comparing the relative reflected magnetic field strengths at each of the PDCs, the position of the implant relative to the external charger can be determined. Audio and/or visual feedback can then be communicated to the patient to allow the patient to improve the alignment of the charger.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. patent application Ser. No. 13/653,185,filed Oct. 16, 2012, which was a continuation of U.S. patent applicationSer. No. 12/579,740, filed Oct. 15, 2009 (now U.S. Pat. No. 8,311,638),which are incorporated herein by reference in its entirety and to whichpriority are claimed. This application is also related tocommonly-assigned U.S. Pat. No. 8,473,066, which is hereby incorporatedby reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to techniques for providing improvedalignment between an external charger and an implantable device.

BACKGROUND

Implantable stimulation devices generate and deliver electrical stimulito body nerves and tissues for the therapy of various biologicaldisorders, such as pacemakers to treat cardiac arrhythmia,defibrillators to treat cardiac fibrillation, cochlear stimulators totreat deafness, retinal stimulators to treat blindness, musclestimulators to produce coordinated limb movement, spinal cordstimulators to treat chronic pain, cortical and deep brain stimulatorsto treat motor and psychological disorders, and other neural stimulatorsto treat urinary incontinence, sleep apnea, shoulder subluxation, etc.The present invention may find applicability in all such applications,although the description that follows will generally focus on the use ofthe invention within a Spinal Cord Stimulation (SCS) system, such asthat disclosed in U.S. patent application Ser. No. 11/177,503, filedJul. 8, 2005.

Spinal cord stimulation is a well-accepted clinical method for reducingpain in certain populations of patients. An SCS system typicallyincludes an Implantable Pulse Generator (IPG), electrodes, at least oneelectrode lead, and, optionally, at least one electrode lead extension.As shown in FIG. 1, the electrodes 106, which reside on a distal end ofthe electrode lead 102, are typically implanted along the dura 70 of thespinal cord 19, and the IPG 100 generates electrical pulses that aredelivered through the electrodes 106 to the nerve fibers within thespinal column 19. Electrodes 106 are arranged in a desired pattern andspacing to create an electrode array 110. Individual wires 112 withinone or more electrode leads 102 connect with each electrode 106 in thearray 110. The electrode lead(s) 102 exit the spinal column 19 and mayattach to one or more electrode lead extensions 120. The electrode leadextensions 120, in turn, are typically tunneled around the torso of thepatient to a subcutaneous pocket where the IPG 100 is implanted.Alternatively, the electrode lead 102 may directly connect with the IPG100.

As should be obvious, an IPG needs electrical power to function. Suchpower can be provided in several different ways, such as through the useof a rechargeable or non-rechargeable battery or through electromagnetic(EM) induction provided from an external charger, or from combinationsof these and other approaches, which are discussed in further detail inU.S. Pat. No. 6,553,263 (“the '263 patent”). Perhaps the favorite ofthese approaches is to use a rechargeable battery in the IPG, such as alithium-ion battery or a lithium-ion polymer battery. Such arechargeable battery can generally supply sufficient power to run an IPGfor a sufficient period (e.g., a day or more) between recharging.Recharging can occur through the use of EM induction, in which EM fieldsare sent by an external charger to the IPG. Thus, when the battery needsrecharging, the patient in which the IPG is implanted can activate theexternal charger to transcutaneously (i.e., through the patient's flesh)charge the battery (e.g., at night when the patient is sleeping orduring other convenient periods).

The basics of such a system are shown in FIG. 2. As shown, the systemcomprises, in relevant part, the external charger 208 and IPG 100. Aprimary coil 130 in the charger 208 produces an EM field 290 capable oftranscutaneous transmission through a patient's flesh 278. The externalcharger 208 may be powered by any known means, such as via a battery orby plugging into a wall outlet, for example. The EM field 290 is met atthe IPG 100 by another coil 270, and accordingly, an AC voltage isinduced in that coil 270. This AC voltage in turn is rectified to a DCvoltage at a rectifier 682, which may comprise a standard bridgecircuit. (There may additionally be data telemetry associated with theEM field 290, but this detail is ignored as impertinent to the presentdisclosure). The rectified DC voltage is, in turn, sent to a chargecontroller and protection circuit 684, which operates generally toregulate the DC voltage and to produce either a constant voltage orconstant current output as necessary for recharging the battery 180.

FIG. 3 shows further details of external charger 208 with the topportion of the housing removed. Further details concerning externalchargers can be found in U.S. patent application Ser. No. 11/460,955,filed Jul. 28, 2006. As shown in FIG. 3, electrical current 114 flowingin a counterclockwise direction through the primary coil 130 induces amagnetic field 290 having a prominent portion in a directionperpendicular to the plane in which the primary coil 130 lies. Primarycoil 130 is typically formed of many turns of copper Litz wire, but theindividual turns are not shown in FIG. 3 for clarity. Thus, when a faceof the case of the external charger 208 is oriented in close proximityto an implanted device, such that the primary coil 130 is parallel to acorresponding coil within the IPG 100, the magnetic field generated bythe primary coil 130 induces an electrical current within acorresponding coil to charge a battery within, or otherwise providepower, to the IPG 100.

This system is akin to a transformer where the primary coil is in theexternal charger 208 and secondary coil in the IPG 100. The efficiencyof this coupling is largely dependent upon the alignment between the twocoils, which efficiency can be expressed as a coupling factor, k.Achieving a good coupling factor is essential for optimizing efficiencyof the inductive link. Not only does good coupling increase the powertransferred to the implant, it minimizes heating in the implant, andalso reduces the power requirements of the external charger, whichreduces heating of the charger and allows a smaller form factor. Propercoupling is also essential if there is to be any data telemetry betweenthe external charger 208 and the implant.

Operation of the external charger 208 in the prior art typicallyinvolves the use of audio feedback to the user. Thus, when chargingbegins, the external charger 208 produces induced field 290 and beginssearching for the IPG 100, as will be explained in more detail herein.An audio transducer in the external charger 208 would provide anintermittent audible sound (e.g., beeping) when coupling was poorbetween the charger 208 and the IPG 100, which beeping would alert theuser to move the external charger relative to the IPG. Once thepositioning and coupling were improved, the charger 208 would stopbeeping, and the location of the charger 208 would be held in place overthe IPG 100 by using double-side adhesive pads or a belt. If the charger208 again became poorly positioned relative to the IPG 100, the audiotransducer would again start beeping, so that the position of thecharger 208 relative to the IPG 100 could again be readjusted. Aback-telemetry link from the IPG 100 would communicate to the charger208 when the IPG battery was fully charged, which condition can again beaudibly signaled to the patient.

As noted earlier, proper alignment between an external charger and animplant is essential for proper system function, energy transfer, andsafety to the patient. However, this has heretofore been difficult toachieve. In particular, it has been noticed by the inventors that it isdifficult for prior art external chargers to differentiate between adeeply-implanted device that is well aligned with respect to thecharger, and a shallowly-implanted device that is poorly aligned withrespect to the charger. Either scenario appears the same to the externalcharger 208. As a result, the patient will only know that the couplingis poor, but will not know how to remedy this situation apart fromtrial-and-error re-positioning of the charger.

Given these shortcomings, the art of implantable devices would benefitfrom techniques for achieving improved coupling between an externalcharger and an implantable device that provide: the ability toaccurately indicate the relative position of the charger to the implant;increased charging efficiency; faster charging rates; increased patientsafety and comfort; lower power requirements; and a smaller form factor.This disclosure presents a solution to this problem involving the use ofposition determination coils that actively induce their own magneticfields.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an implantable pulse generator (IPG), an external charger,and the manner in which an electrode array is coupled to the IPG, inaccordance with the prior art.

FIG. 2 illustrates a prior art system comprising an external charger forcharging an implantable pulse generator, including the charge controllerand battery protection aspects of the IPG.

FIG. 3 shows a perspective view of a prior art external charger for animplantable medical device.

FIGS. 4A-4C illustrate typical configurations, wherein the primary coilof a prior art external charging device is located at or near the outersurface of the patient's skin and the secondary coil of an implantablemedical device is located near to or far from the inner surface of thepatient's skin.

FIG. 5A shows a perspective view of one possible embodiment of animproved external charger for an implantable medical device wherein theposition determination coils don't overlap with the outline of theimplanted medical device.

FIGS. 5B-5D show the effects on charging coil voltage and positiondetermination coil voltage due to various charger/implant alignmentscenarios.

FIG. 6A shows a perspective view of one possible embodiment of animproved external charger for an implantable medical device wherein theposition determination coils overlap with the outline of the implantedmedical device.

FIGS. 6B-6D show the effects on charging coil voltage and positiondetermination coil voltage due to various charger/implant alignmentscenarios.

FIG. 7A shows a typical configuration, wherein the primary coil of animproved external charging device is located at or near the outersurface of the patient's skin and the secondary coil of an implantablemedical device is located near the inner surface of the patient's skin.

FIG. 7B shows two position determination coils whose outputs are sent toposition indication circuitry.

FIG. 8A is a flowchart detailing one embodiment of a technique forassuring the proper alignment of an external charger to an IPG whereinthe charging coil and the position determination coils are driven atdifferent frequencies.

FIG. 8B is a flowchart detailing one embodiment of a technique forassuring the proper alignment of an external charger to an IPG whereinthe charging coil and the position determination coils are driven at thesame frequency.

FIGS. 9A-9B illustrate typical configurations, wherein the primary coilof an improved external charging device is located at or near the outersurface of the patient's skin and the secondary coil of an implantablemedical device is located near to or far from the inner surface of thepatient's skin.

FIG. 10 shows a system comprising an improved external charger forcharging an implantable pulse generator, including the alignment sensingand position indication circuitry of the external charger.

FIG. 11 shows one embodiment of an improved external charger forcharging an implantable pulse generator.

DETAILED DESCRIPTION

The description that follows relates to use of the invention within aspinal cord stimulation (SCS) system. However, it is to be understoodthat the invention is not so limited. Rather, the invention may be usedwith any type of implantable medical device that could benefit fromimproved alignment between an external charger and the implantabledevice. For example, the present invention may be used as part of asystem employing an external charger configured to charge a pacemaker,an implantable pump, a defibrillator, a cochlear stimulator, a retinalstimulator, a stimulator configured to produce coordinated limbmovement, a cortical or deep brain stimulator, or in any otherstimulator configured to treat urinary incontinence, sleep apnea,shoulder subluxation, etc. Moreover, the technique can be used innon-medical and/or non-implantable devices or systems as well, i.e., inany device or system in which proper coupling between a primary andsecond device is necessary or desirable.

As noted earlier, achieving proper coupling between an external chargerand an implant can be difficult, as it is hard for the external chargerto differentiate between a deep implant that is well aligned to theexternal charger and a shallow implant that is misaligned with theexternal charger. Both scenarios appear the same to the externalcharger. The present invention provides an improved external chargerhaving improved means for determining the position of the implanteddevice relative to the charger by actively inducing one or more magneticfields and measuring the reflected magnetic field from the implanteddevice.

In one embodiment, the external charger 208 contains positiondetermination coils (PDCs) to help discriminate between deep implantsand misaligned implants. Through use of these magnetic field-inducingPDCs, it is possible to determine the position of an implantable deviceby measuring the amount of reflected magnetic field coming from theimplant at each PDC. In one embodiment, a plurality of PDCs are arrangedwithin the charge coil in a plane or planes parallel to the charge coil.By comparing the relative reflected magnetic field strengths at each ofthe PDCs, the position of the implant can be determined. Audio and/orvisual feedback can then be communicated to the patient to allow thepatient to improve the alignment of the charger.

FIG. 4A shows a primary coil 130 configured for transcutaneouslycharging the IPG 100 via inductive coupling in accordance with the priorart. As mentioned earlier, the charger 208 comprises a primary coil 130,through which an AC current 114 is passed via an AC current source 170.This current 114 produces induced magnetic field 290, which isillustrated as a plurality of flux lines 160. Flux lines 160 areessentially perpendicular to the surface of the skin 278 where they passthrough its surface. In addition, the magnetic flux lines 160 near thecenter of the primary coil 130 are substantially parallel to the centralaxis 275 of the coil. A corresponding coil 270 within the IPG 100transforms this magnetic energy into an electrical current, which isrectified and used by circuitry within the IPG 100 to, e.g., charge abattery 180. The distance between the charger 208 and the IPG 100 istypically on the order of about 1-5 centimeters.

The primary and secondary coils 130 and 270 are substantially in theshape of a circular loop, and are typically formed of several turns ofwire, as one skilled in the art will appreciate. However, it will berecognized that the substantially circular shape of the coils 130 and270 are merely illustrative. The turns of the primary coil 130 define acenter opening or aperture having a central axis 275. It will berecognized that the surface of the skin 278 is not always flat. Hence,the central axis 275 of the primary coil 130 is sometimes onlyapproximately or substantially perpendicular to the surface of the skin278.

The induced magnetic field 290 produces eddy currents in the IPG'stypically metallic case 101 or in other conductive structures within theIPG 100. Such eddy currents operate to produce a reflected magneticfield 295, which operates to change the mutual inductance of the primarycoil 130, effectively “detuning” the coil. Such detuning changes Vcoil,the voltage used to produce the current in the primary coil 130.Accordingly, from monitoring Vcoil, the relative coupling between theexternal charger 208 and the IPG 100 can be inferred. Vcoil decreases asthe coupling increases, which generally occurs when the external charger208 and the IPG 100 are closer to one another.

However, this means of monitoring coupling between the external charger208 and the IPG 100 cannot discern between distance and misalignment,which conditions are illustrated in FIGS. 4B and 4C. FIG. 4B shows anIPG 100 implanted relatively deeply within a patient, but otherwise wellaligned from an axial perspective, i.e., coil axes 275 and 276 (see FIG.4A) are not offset from each other. FIG. 4C, by contrast, shows an IPG100 implanted relatively shallowly with a patient, but with pooralignment, i.e., coil axes 275 and 276 (see FIG. 4A) are offset to alarge degree. In either of these cases, the coupling between theexternal charger and the IPG 100 will be relatively poor, with theresult that Vcoil will not be greatly affected by the IPG 100. However,because Vcoil might be the same in magnitude for both conditions, Vcoilcannot be used to discern between depth (FIG. 4B) and misalignment (FIG.4C). As a result, Vcoil cannot be used by the external charger 208—andultimately the patient—to qualify the reason for poor coupling, or howto fix the poor coupling by appropriate repositioning of the externalcharger 208.

FIG. 5A shows one embodiment of an improved external charger 210 withthe ability to determine the relative position of an implanted device,and thus maximize coupling by indicating to the user how to improvecharger/device alignment. In this embodiment, the four PDCs 230 arearranged into two pairs, 230 x 1/230 x 2 and 230 y 1/230 y 2, of twoactive magnetic field inducing coils each. Each PDC is configured toinduce its own magnetic field, labeled as: 290 x 1, 290 x 2, 290 y 1,and 290 y 2. The induced magnetic field of the large charging coil 130is labeled as 290. Each of the pairs of PDCs 230 x and 230 y arepositioned within primary coil 130 and such that the plurality of PDCsare wound around axes that are parallel to the central axis 275 (FIG.7A).

The PDCs 230 are designed to induce magnetic fields that aresubstantially co-axial with their central axes when placed near thesurface of the skin 278. By detecting the amount of “detuning,” i.e.,the decrease in voltage, across each of the PDCs due to the reflectedmagnetic field 295 caused by the presence of an IPG 100 under the skin'ssurface, the position of the IPG 100 can be inferred. Each pair of PDCs230 x and 230 y may straddle the central axis 275 (FIG. 7A) of theprimary coil 130, such that the coils in each pair are equidistant fromthe central axis 275 and opposite each other. As shown, the pairs 230 xand 230 y are positioned orthogonally with respect to each other.

Element 101 represents the outline of an IPG that would be implantedwithin the patient's body. In FIG. 5A, the PDCs 230 are of a smallenough size that, when an IPG 100 is perfectly centered within them, thePDCs 230 would only be minimally detuned by the presence of the IPG 100.For example, when the charging coil 130 itself was “detuned” by thepresence of the IPG 100, it would be able to indicate to the externalcharger 210's position indication circuitry 279 (FIG. 7B) that there wasan implant somewhere under the surface of the skin in the vicinity ofthe external charger 210. At that point, the external charger 210 couldactivate the PDCs 230 to “fine tune” the alignment of the externalcharger 210 with the IPG 100. If the voltage of each of the PDCs 230 is10V, for example, the external charger 210 would indicate that it wasproperly aligned with the IPG 100 when each of the PDCs 230 weresimultaneously minimally “detuned” by substantially the same amount,e.g., 2V. If the external charger 210 were misaligned from IPG 100 inany direction, the PDCs 230 that were disproportionally closer to theIPG 100 would, due to the “detuning” effect, register lower voltagesthan the PDCs 230 that were further away from the IPG 100. By comparingthe voltages across each of the PDCs 230, position indication circuitry279 (FIG. 7B) can indicate to the user the direction that the externalcharger 210 should be moved in to improve alignment.

FIGS. 5B-5D show in graphical form the “detuning” effects on the variouscoils of external charger 210 when an IPG 100 is placed in front of theexternal charger 210 of FIG. 5A. In FIG. 5B, there is no IPG 100 in thevicinity of the external charger 210's primary coil 130. Thus, there isno detuning of the primary coil 130 and only minimal detuning of thePDCs 230. As such, Vcoil will register as whatever voltage the primary,i.e., charging, coil 130 is being driven at. In graph 400 a, this isshown to be 140 volts. Likewise, the PDCs—VcoilX1, VcoilX2, VcoilY1, andVcoilY2—will register at nearly the voltage they are being driven at. Ingraph 400 a, this is shown to be 10 volts. These conditions would beinterpreted by position indication circuitry 279 (FIG. 7B) to mean thatthere was no IPG 100 present.

The dashed lines 290 around each PDC 230 represent roughly the reach ofthe induced magnetic field of the respective PDC 230. For a PDC 230 witha radius of r, within 2r distance from the PDC 230's center, the PDCmagnetic field 290 is especially strong, i.e., it can easily be detectedby measurement circuitry or affected by the presence of other magneticfields. Therefore, if an IPG 100 is present anywhere within 2r distancefrom the center of the PDC 230, the PDC 230's induced magnetic field maybe appreciably detuned, indicating the presence of an implanted deviceto position indication circuitry 279. Similarly, if two PDCs 230 arecloser than 2r distance to each other, each one's induced magnetic fieldmay be affected by the other's, i.e., there may be “cross talk” betweenthe PDCs 230.

In FIG. 5C, the IPG 100 is perfectly aligned within external charger210's primary coil 130, shown by IPG outline 101. As such, Vcoil willregister as being lower than the voltage that the primary, i.e.,charging, coil 130 is being driven at due to the detuning effect of theIPG 100. In graph 400 b, the charging coil is shown to be detuned from avalue of 140 volts to a value of 100 volts, indicating the presence ofan implanted IPG 100 in the vicinity of external charge 210. However,the PDCs—VcoilX1, VcoilX2, VcoilY1, and VcoilY2—would register aslightly lower voltage than they were in FIG. 5B. In graph 400 b, thisis shown to be 8 volts. This occurs because the IPG 100 lies completelywithin the PDCs 230, and thus the magnetic fields induced by the PDCs230 are reflected back by the IPG 100, but not as significantly as theywould be if there were a greater overlap between the PDCs 230 and theIPG 100. These conditions would be interpreted by position indicationcircuitry 279 (FIG. 7B) to mean that an IPG 100 was both present andwell-aligned with respect to the primary coil 130. In FIG. 5C, therewould be almost no “cross talk” from one PDC 230 to another due to thePDC's relatively small radii, as is seen by the lack of overlapping ofthe dashed magnetic field lines 290.

In FIG. 5D, the IPG 100 is in the vicinity of external charger 210'sprimary coil 130, but is misaligned, as is shown by IPG outline 101. Assuch, Vcoil will register as being lower than the voltage that theprimary, i.e., charging, coil 130 is being driven at due to the detuningeffect of the IPG 100. In graph 400 c, the charging coil is shown to bedetuned from a value of 140 volts to a value of 100 volts, indicatingthe presence of an implanted IPG 100 in the vicinity of external charge210. Two of the PDCs—VcoilX2 and VcoilY2—may register at slightly lessthan 10V. In graph 400 c, this is shown to be 9 volts. However, two ofthe PDCs—VcoilX1 and VcoilY1—have been detuned to a value of 6 volts.This occurs because the IPG 100 is closer to PDCs 230 x 1 and 230 y 1than it is to the other PDCs. Thus, the magnetic fields induced by PDCs230 x 1 and 230 y 1 are reflected back by the IPG 100 and detune thesecoils to some extent. These conditions would be interpreted by positionindication circuitry 279 (FIG. 7B) to mean that an IPG 100 was presentbut misaligned with respect to the primary coil 130, specifically thatthe IPG 100 was closer to coils 230 x 1 and 230 y 1. External charger210 could then indicate to the user to move the external charger 210downwardly and to the left to improve charging alignment.

In FIG. 6A, the PDCs are depicted as being large enough that they wouldoverlap with the outline 101 of an IPG centered perfectly within them inthe body. This would likewise produce the effect that each of the PDCswould be “detuned” in equal amounts by the presence of a perfectlyaligned IPG 100. For example, when the charging coil 130 itself was“detuned” by the presence of the IPG 100, it would be able to indicateto the external charger 210's position indication circuitry 279 (FIG.7B) that there was an implant somewhere under the surface of the skin inthe vicinity of the external charger 210. At that point, the externalcharger 210 could activate the PDCs 230 to “fine tune” the alignment ofthe external charger 210 with the IPG 100. If each of the PDCs 230 werebeing driven at 10V, for example, the external charger 210 wouldindicate that it was properly centered over the IPG 100 when each of thePDCs 230 were simultaneously “detuned” by an equal amount, e.g., each ofthe PDCs 230 may register a voltage of 7V. If the external charger 210were misaligned from IPG 100 in any direction, the PDCs 230 that weredisproportionally closer to the IPG 100 would, due to the “detuning”effect, register disproportionally lower voltages than the PDCs 230 thatwere further away from the IPG 100. By comparing the voltages acrosseach of the PDCs 230, position indication circuitry 279 (FIG. 7B) canindicate to the user the direction that the external charger 210 shouldbe moved in to improve alignment.

FIG. 6B-6D show in graphical form the “detuning” effects on the variouscoils of external charger 210 when an IPG 100 is placed in front of theexternal charger 210 of FIG. 6A. In FIG. 6B, there is no IPG 100 in thevicinity of the external charger 210's primary coil 130. Thus, there isno detuning of either the primary coil 130 or the PDCs 230. As such,Vcoil will register as whatever voltage the primary, i.e., charging,coil 130 is being driven at. In graph 410 a, this is shown to be 140volts. Likewise, the PDCs—VcoilX1, VcoilX2, VcoilY1, and VcoilY2—willregister as approximately the voltage they are being driven at, withsome deviation possible due to potential cross talk between the PDCs. Ingraph 410 a, this is shown to be 10 volts. These conditions would beinterpreted by position indication circuitry 279 (FIG. 7B) to mean thatthere was no IPG 100 present.

In FIG. 6C, by contrast, the IPG 100 is perfectly aligned withinexternal charger 210's primary coil 130, shown by IPG outline 101. Assuch, Vcoil will register as being lower than the voltage that theprimary, i.e., charging, coil 130 is being driven at due to the detuningeffect of the IPG 100. In graph 410 b, the charging coil is shown to bedetuned from a value of 140 volts to a value of 100 volts, indicatingthe presence of an implanted IPG 100 in the vicinity of external charge210. The PDCs—VcoilX1, VcoilX2, VcoilY1, and VcoilY2—in graph 410 b areeach registering a voltage less than they are being driven at. In graph410 b, this is shown to be 7 volts. This occurs because the IPG 100 iswell-aligned with primary coil 130, and thus, each of the magneticfields induced by the PDCs 230 are reflected back in equal amounts bythe IPG 100, causing a substantially equal amount of detuning in eachPDC 230. These conditions would be interpreted by position indicationcircuitry 279 (FIG. 7B) to mean that an IPG 100 was both present andwell-aligned with respect to the primary coil 130. Due to thepotentially asymmetrical nature of the IPG 100, the PDCs 230 may noteach be detuned to exactly the same voltage when the IPG 100 isperfectly aligned. However, position indication circuitry 279 may storesome threshold values at which it will determine that each of the PDCsis detuned by a sufficiently equal amount that the IPG 100 is wellaligned with the primary coil 130.

In FIG. 6D, the IPG 100 is in the vicinity of external charger 210'sprimary coil 130, but is misaligned, as is shown by IPG outline 101. Assuch, Vcoil will register as being lower than the voltage that theprimary, i.e., charging, coil 130 is being driven at due to the detuningeffect of the IPG 100. In graph 410 c, the charging coil is shown to bedetuned from a value of 140 volts to a value of 100 volts, indicatingthe presence of an implanted IPG 100 in the vicinity of external charge210. Two of the PDCs—VcoilX2 and VcoilY2—are still registering a voltageonly minimally detuned from the voltage they are being driven at. Ingraph 410 c, this is shown to be 8 volts. However, two of thePDCs—VcoilX1 and VcoilY1—have been detuned to a value of 5 volts. Thisoccurs because the IPG 100 is closer to PDCs 230 x 1 and 230 y 1 than itis to the other PDCs. Thus, the magnetic fields induced by PDCs 230 x 1and 230 y 1 are reflected back by the IPG 100 and detune these coils tosome extent. These conditions would be interpreted by positionindication circuitry 279 (FIG. 7B) to mean that there was an IPG 100 waspresent but misaligned with respect to the primary coil 130,specifically that the IPG 100 was closer to coils 230 x 1 and 230 y 1.External charger 210 could then indicate to the user to move theexternal charger 210 downwardly and to the left to improve chargingalignment.

In FIGS. 6C and 6D, there may be some “cross talk” from one PDC 230 toanother due to the PDC's relatively larger radii when compared to theembodiment shown in FIGS. 5A-5D, as is seen by the overlapping of thedashed magnetic field lines 290. One possible solution to this potentialissue would be to adjust the expected voltages from each PDCs 230 whenno IPG 100 is present. For example, through experimentation, it may befound that the cross talk with a perfectly centered IPG 100 may resultin a baseline PDC voltage of 9V rather than 10V as is shown in FIG. 6B.Another solution to this issue may be to enable one PDC 230 at a time,rapidly cycling through each PDC, sensing the voltage across each PDC asit is enabled (See FIG. 8A).

Although it may be more difficult to determine when an external charger210 is perfectly aligned using the embodiment of FIG. 6A than it is withthe embodiment of FIG. 5A due to, e.g., the asymmetrical nature of theIPG 100, it may be necessary to increase the diameter of the PDCs, as isshown in FIG. 6A when compared to FIG. 5A, to achieve a satisfactorysignal-to-noise ratio when alignment sensing circuitry 281 is attemptingto measure the amount of “detuning” in the PDCs 230. Increasing thediameter of the PDCs 230 also allows the external charger 210 to detecta misaligned IPG 100 at a further distance from the charger, e.g.,implanted in the body at a greater depth. However, increasing thediameter of the PDCs may also increase the amount of “cross talk”between each of the individual PDC coils 230.

FIG. 7A shows an improved external charger 210 with a primary coil 130configured for transcutaneously charging the IPG 100 via inductivecoupling and with PDCs 230 arranged in accordance with the embodimentshown in FIG. 6A. The improved external charger 210 comprises a primarycoil 130, through which an AC current 114 is passed via an AC currentsource 170 at a frequency, f_(coil). This current 114 produces inducedmagnetic field 290 (FIGS. 4A-4C). Improved external charger 210 furthercomprises a plurality of PDCs 230, through which an AC current 115 ispassed via a plurality of AC current sources 171 a-d at a frequency,f_(PDC). In FIG. 7A, only a single AC current source is shown forsimplicity. It is possible, and may be preferable, to drive each PDC 230using a separate AC current source 171. However, the present disclosurealso contemplates a system in which each of the PDCs 230 could be drivenby a single AC current source 171 whose signal is fanned out to each ofthe PDCs 230. Current 115 from current sources 171 a-d produces inducedmagnetic fields 290 x 1, 290 x 2, 290 y 1, 290 y 2 (FIGS. 5A, 6A), whichare illustrated as a plurality of flux lines 160. By comparingelectrical measurements, such as the amount of the “detuning” in thePDCs 230 due to the reflected magnetic field 295 passing through them,the position of the implant in both the x- and y-directions can bedetermined by the external charger 210's position indication circuitry279 (FIG. 7B). Audio and/or visual feedback of the implant position canthen be communicated to the patient to improve alignment of the charger.

FIG. 7B shows one potential arrangement of a pair of PDCs 230 x for theimproved external charger 210 that is depicted in FIGS. 5A and 6A. Inthis embodiment, only PDCs 230 x 1 and 230 x 2, which are used todetermine the IPG 100's misalignment with the external charger 210 inthe x-direction (and not PDCs 230 y 1 and 230 y 2), are shown for thesake of simplicity. A complete external charger 210 utilizing thisembodiment will also have a corresponding pair of PDCs 230 y 1 and 230 y2 to measure misalignment in the y-direction. In the embodiment of FIG.7B, each of the PDCs 230 x 1 and 230 x 2 are driven by their own currentsource 171. Further, each PDC in a PDC pair has one terminal connectedto ground and the other terminal connected to a simple voltage dividercircuit 274 that serves to divide down the voltage measured across thecoil before sending the measured voltage value to position indicationcircuitry 279. As an alternative to measuring voltages, the positionindication circuitry 279 could independently sense the strength of thecurrent 115 passing through each PDC 230 to determine the amount ofdetuning in each PDC 230.

In the embodiment of FIG. 7B, the voltages (or currents) across each PDCwithin each PDC pair are compared to each other within the positionindication circuitry 279 in order to determine the misalignment of theexternal charger 210 with respect to a particular direction. In otherwords, alignment sensing circuitry 281 derives a first indicator and asecond indicator, wherein the first and second indicators indicatemisalignment with respect to first and second directions. Positionindication circuitry 279 can then determine the location of theimplantable medical device 100 and deliver appropriate instruction tothe user as to how to improve the external charger 210's alignment withthe implantable medical device 100.

For example, if IPG 100 is closer in the x-direction to PDC 230 x 1 thanit is to PDC 230 x 2, the voltage detected at PDC 230 x 1 will be lower,say 8V, than the voltage detected at PDC 230 x 2, say 10V. In this case,there would be a difference of positive two volts (VcoilX2−VcoilX1). Ifinstead, the IPG 100 is closer in the x-direction to PDC 230 x 2, thevoltage at PDC 230 x 1 will be higher, say 10V, than the voltagedetected at PDC 230 x 2, say 8V. In this case, there would be adifference of negative two volts. The magnitude of the differencebetween VcoilX1 and VcoilX2 also indicates relative closeness of theprimary coil 130 and the IPG 100. For example, if the voltages measuredat 230 x 1 and 230 x 2 were 2V and 10V, respectively, instead of 8V and10V as in the example above, the difference between the signals would be8V. The greater magnitude of difference would indicate to positionindication circuitry 279 that IPG 100 was located even further towardsPDC 230 x 1 in the 2V/10V scenario than it was in the 8V/10V scenario.Thus, this embodiment is able to provide detailed information about theIPG 100's relative location in the x-direction. As will be understood,the same processing can simultaneously be carried out by PDCs 230 y 1and 230 y 2 to determine the IPG 100's relative location in they-direction, thus allowing external charger 210 to give a completepicture of IPG 100's location.

The embodiments of FIGS. 7A-7B show that the primary coil 130 is drivenby an AC voltage of V_(coil) at a frequency of f_(coil), whereas thePDCs 230 are driven at a voltage of V_(PC) at a frequency of f_(PDC). Incertain embodiments, it will be advantageous to drive the charging coil130 and PDCs 230 at different frequencies. Because the field produced bythe charging coil 130 is more powerful than the field produced by anindividual PDC 230, if f_(coil) were equal to f_(PDC), it might bedifficult for alignment sensing circuitry 281 to simultaneouslydetermine the coil voltage at any of the PDCs 230 while the charge coil130 is active because the charge coil 130's voltage might block out or“flood” the PDCs. Thus, in some embodiments, the charging coil 130 couldbe driven at, e.g., 140V and 80 kHz, whereas the PDCs 230 could bedriven at, e.g., 10V and 800 kHz-1 MHz. The expected current draw of thecharging coil 130 may be approximately 300 mA, whereas the expectedcurrent draw of each of the PDCs 230 might be 25 mA. Thus, the externalcharger 210 could possess at least two distinct signals sources—onesignal for the charging coil 130, and one or more signals that can berouted to each of the PDCs 230. The charging coils are ideally driven atthe smallest voltage possible so that the external charger 210's powersource 180 expends a minimal amount of energy in powering them. It isalso advantageous to ensure that the charging coil 130 and the PDCs 230are driven at frequencies that are not direct integer harmonics of eachother so that there is no interference when measuring the various coilvoltages.

FIG. 8A is a flowchart detailing one embodiment of a technique forassuring the proper alignment of an external charger 210 to an IPG 100,wherein the charging coil and the PDCs are driven by AC power sources atdifferent, non-integer harmonic, frequencies—that is, f_(coil) does notequal f_(PDC). First, the user places external charger 210 against thesurface of his body 278 in the known vicinity of IPG 100 (310). At thistime, the patient will activate the external charger 210 and begincharging IPG 100 (320). The default setting for external charger 210 ismaximum power output. As long as external charger doesn't receive anindication that IPG 100 is fully charged (330), it will continue tocharge IPG 100. As external charger 210 is charging IPG 100 at afrequency of f_(coil), alignment sensing circuitry 281 in the externalcharger 210 senses the charger's alignment with the IPG 100 based atleast in part on electrical measurements taken from the plurality ofPDCs 230 actively inducing magnetic fields at a frequency of f_(PDC),and position indication circuitry 279 calculates the IPG 100's location(340). This calculation (340) occurs in real time so that, any timealignment becomes poor, corrective action can be indicated to the userand taken in subsequent steps. If IPG 100 and the external charger 210are properly aligned (350), external charger 210 continues to charge theIPG 100's internal power source 180 until receiving indication that IPG100 is fully charged (330). If the external charger 210 determines thatIPG 100 and the external charger 210 are not properly aligned (350), theexternal charger 210 will indicate to the user which direction to movethe external charger 210 to improve alignment (360) while stillcontinuing to charge IPG 100. Once the external charger 210 determinesthat the IPG 100's internal power source 180 is fully charged (330), itwill indicate via an audible beep or other visual indication to the userthat the charging process has completed (370).

In other embodiments, it may be possible to drive the charging coil 130and PDCs 230 at the same frequency. Because the field produced by thecharging coil 130 is much more powerful than the field produced by anindividual PDC 230, if f_(coil) were to equal f_(PDC), it might bedifficult for alignment sensing circuitry 281 to simultaneouslydetermine the coil voltage at any of the PDCs 230 while the charge coil130 is active due to the charge coil 130's voltage blocking out or“flooding” the PDCs. Thus, in some embodiments, the charging coil 130could be temporarily shut off to allow alignment sensing circuitry 281to either sequentially or simultaneously measure the voltages at each ofthe PDCs 230 and send the measurements to the position indicationcircuitry 279. For example, the charging coil 130 could be deactivatedand the PDCs 230 could be queried for 50-100 ms out of each second, withthe charging coil activated for the remaining 900-950 milliseconds outof each second. In such an embodiment, the external charger 210 couldpossess a single AC signal source for driving the charging coil 130 andthe PDCs 230 at a single frequency. The charging coils are ideallydriven at the smallest voltage possible so that the external charger210's power source 180 expends a minimal amount of energy in poweringthem.

FIG. 8B is a flowchart detailing one embodiment of a technique forassuring the proper alignment of an external charger 210 to an IPG 100,wherein the charging coil and the PDCs are driven by AC power sources atthe same frequency—that is, f_(coil) equals f_(PDC). First, the userplaces external charger 210 against the surface of his body 278 in theknown vicinity of IPG 100 (310). At this time, the patient will activatethe external charger 210 and begin charging IPG 100 (320). The defaultsetting for external charger 210 is maximum power output. As long asexternal charger doesn't receive an indication that IPG 100 is fullycharged (330), it will continue to charge IPG 100. For a predeterminedtime interval, external charger 210 will temporarily disable chargingcoil 130 so that it can take measurements from the PDCs 230 (335). Whileexternal charger 210 has temporarily ceased charging IPG 100 at afrequency of f_(coil), alignment sensing circuitry 281 in the externalcharger 210 senses the charger's alignment with the IPG 100 based atleast in part on electrical measurements taken from the plurality ofPDCs 230 actively inducing magnetic fields at a frequency of f_(coil),and position indication circuitry 279 calculates the IPG 100's location(345). This calculation (345) occurs in real time so that, any timealignment becomes poor, corrective action can be indicated to the userand taken in subsequent steps. If IPG 100 and the external charger 210are properly aligned (350), external charger 210 resumes charging theIPG 100's internal power source 180 until receiving indication that IPG100 is fully charged (330). If the external charger 210 determines thatIPG 100 and the external charger 210 are not properly aligned (350), theexternal charger 210 will indicate to the user which direction to movethe external charger 210 to improve alignment (360) while still resumingits charging of IPG 100. Once the external charger 210 determines thatthe IPG 100's internal power source 180 is fully charged (330), it willindicate via an audible beep or other visual indication to the user thatthe charging process has completed (370).

FIG. 9A shows a scenario where an implantable medical device 100 isdeeply implanted in the patient's body but well aligned with externalcharger 210. In this scenario, the primary coil would be slightlydetuned by the presence of the IPG (e.g., detuned from 140V to 120V),and each of the PDCs 230 would have a similar Vcoil because each coilwould pick up an equivalent reflected magnetic flux, i.e., VcoilX1,VcoilX2, VcoilY1, and VcoilY2 would all be equal, as is shown in graph420 a of FIG. 9A (e.g., each is detuned from 10V to 8V). Thus, thedifferences between the measured voltages of the two PDCs in each of thePDC pairs would be zero or close to zero, and the position indicationcircuitry 279 would determine that the external charger 210 wasproperly, i.e., symmetrically, aligned with the IPG 100.

FIG. 9B shows a scenario where an implantable medical device 100 isshallowly implanted in the patient's body but poorly aligned withexternal charger 210; specifically, it is skewed in the y-direction. Asdiscussed above, a prior art external charger would not be able todistinguish between the scenario presented in FIG. 9A and the scenariopresented in FIG. 9B because the only measurement available would beVcoil, which is 120V in both scenarios. However, with the improvedexternal charger 210, these two scenarios are distinguishable. In thescenario shown in FIG. 9B, field PDC 230 y 2 measures a lower Vcoil(VcoilY2) than the other PDCs because PDC 230 y 2 picks up adisproportionately larger amount of reflected magnetic flux, as is shownin graph 420 b of FIG. 9B (e.g., VcoilY2 is detuned from 10V to 6V,VcoilY1 is not detuned at all, and VcoilX1 and VcoilX2 are onlyminimally detuned). As was discussed above, the alignment sensingcircuitry 281 can compare the VcoilY1 and VcoilY2 values. In thisscenario, it would result in a determination that VcoilY2 is smallerthan VcoilY1 and alignment sensing circuitry 281 would send signals toposition indication circuitry 279 that would be interpreted to mean thatthe IPG 100 was actually closer to PDC 230 y 2 than it was to PDC 230 y1, i.e., that the charger 210 was too far to the left as illustrated.The external charger 210 would then indicate to the user how to correctthe alignment problem, i.e., by instructing the user to move the charger210 to the right, to maximize the electrical coupling of externalcharger 210 and implantable medical device 100. The same processing issimultaneously carried out by PDCs 230 x 1 and 230 x 2 to reportinformation about the IPG 100's location in the x-direction.

FIG. 10 shows a block diagram of an improved alignment detection systemcomprising an improved external charger 210 for inducing a magneticfield, including magnetic field-inducing PDCs 230, alignment sensingcircuitry 281 for measuring reflections of the induced magnetic fields,and the position indication circuitry 279. The implantable device'scircuitry 228 is similar to that described in reference to FIG. 2 above,and is shown in a block for simplicity. Alignment sensing circuitry 281comprises the circuitry for reading the PDCs 230 and may be affixed tothe printed circuit board (PCB) of the external charger 210. Alignmentsensing circuitry 281 sends the PDC information to the positionindication circuitry 279, which discerns the alignment between theimplanted device 100 and the external charger 210. Position indicationcircuitry 279 then indicates to the user a direction in which theexternal charger 210 should be moved to improve the alignment of theexternal charger 210 relative to the implantable medical device 100.Such indication may occur in a variety of ways, including, but notlimited to: activating visual indicators, such as LED lights 293 whichcan be configured to light up on the surface of the external charger 210(See FIG. 11); activating audible indicators, such as beeps or verbalcommands to the user; or activating tactile indicators, such asvibrating certain sides of the external charger 210 to indicate that theexternal charger 210 needs to be moved in that direction.

Because external charger 210 is often placed against a patient's back orbuttocks, it can be difficult for the patient to receive informationfrom the external charger 210 indicating how to improve the charger'salignment. To provide better positioning information to the patient, theexternal charger 210 may optionally transmit, via communications link250, misalignment information to another external device for controllingthe therapeutic settings of the implantable medical device, e.g., remotecontrol 218. The external device may then indicate how the externalcharger 210 should be moved to improve the alignment of the externalcharger 210 relative to the implantable medical device 100. This type ofcommunication is disclosed in commonly-owned U.S. patent applicationSer. No. 12/476,523, filed Jun. 2, 2009.

FIG. 11 shows one embodiment of an improved external charger 210 forcharging an implantable device. The external charger 210 is shownsitting in a base unit 296. In this embodiment, four arrow-shaped LEDlights 293 are arranged on the surface of the external charger 210, withone arrow-shaped LED light pointing towards each edge of externalcharger 210. As position indication circuitry 279 determines in whichdirection the external charger 210 should be moved to provide betteralignment with implantable device 100, it can send an appropriatecontrol signal to illuminate one or more of the LED lights 293 toindicate that direction to the user. When position determinationcircuitry 279 has detected that there is a satisfactory degree ofalignment between the external charger 210's primary coil 130 and theimplantable device, position indication circuitry 279 will send acontrol signal to turn off each LED light 293 until it again senses amisalignment condition during charging.

Although particular embodiments of the present invention have been shownand described, it should be understood that the above discussion is notintended to limit the present invention to these embodiments. It will beobvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present invention. Thus, the present invention is intended to coveralternatives, modifications, and equivalents that may fall within thespirit and scope of the present invention as defined by the claims.

What is claimed is:
 1. An external charger for providing power to an implantable medical device, comprising: one or more current or voltage sources configured to provide a plurality of currents that produce a plurality of magnetic fields at different locations in the external charger; and an alignment sensing circuit configured to determine an alignment of the external charger relative to the implantable medical device, wherein the determination is based on electrical measurements for each of the plurality of currents taken from the one or more current or voltage sources.
 2. The external charger of claim 1, further comprising a position indication circuit configured to receive the determination and to indicate a misalignment of the external charger relative to the implantable medical device.
 3. The external charger of claim 2, wherein the position indication circuit is further configured to indicate how to improve the alignment of the external charger relative to the implantable medical device.
 4. The external charger of claim 2, wherein the position indication circuit is configured to activate visual indicators on the external charger.
 5. The external charger of claim 4, wherein the visual indicators indicate a direction in which the external charger should be moved to improve the alignment of the external charger relative to the implantable medical device.
 6. The external charger of claim 1, wherein the electrical measurements are indicative of reflections of the magnetic fields from the implantable medical device.
 7. The external charger of claim 1, wherein the one or more current or voltage sources comprise one or more current sources, and wherein the electrical measurements comprises voltages used by the one or more current sources to provide each of the plurality of currents.
 8. The external charger of claim 1, wherein the one or more current or voltage sources comprise one or more voltage sources, and wherein the electrical measurements comprises the magnitude of the plurality of currents provided by the one or more voltage sources.
 9. The external charger of claim 1, further comprising a primary coil, wherein the primary coil is configured to produce a primary coil magnetic field to provide power to the implantable medical device, wherein a central axis of the primary coil magnetic field and central axes of the plurality of magnetic fields are parallel.
 10. The external charger of claim 9, wherein the primary coil magnetic field has a first frequency, the plurality of magnetic fields have second frequencies, and the first frequency is different from the second frequency.
 11. The external charger of claim 9, wherein the central axes of the plurality of magnetic fields are equidistant from the central axis of the primary coil magnetic field at the different locations.
 12. The external charger of claim 1, wherein the plurality of currents are provided to a plurality of coils at the locations.
 13. The external charger of claim 1, comprising a plurality of current or voltage sources, each configured to provide one of the plurality of currents.
 14. The external charger of claim 1, comprising a single current or voltage source for providing each of the plurality of currents.
 15. A method for providing power to an implantable medical device using an external charger, comprising: providing a first magnetic field from the external charger to provide power to the implantable medical device; providing from one or more current or voltage sources a plurality of currents that produce a plurality of second magnetic fields at different locations in the external charger; and determining an alignment of the external charger relative to the implantable medical device, wherein the determination is based on electrical measurements taken from the one or more current or voltage sources.
 16. The method of claim 15, wherein determining the alignment comprises determining in which direction to move the external charger to improve the alignment of the external charger relative to the implantable medical device.
 17. The method of claim 16, further comprising transmitting to another external device the determined direction.
 18. The method of claim 17, wherein the another external device comprises an external device for controlling the therapeutic settings of the implantable medical device.
 19. The method of claim 16, further comprising indicating the determined direction to a user.
 20. The method of claim 15, wherein the locations occur in pairs, wherein the electrical measurements taken from the one or more current or voltage sources providing the currents to a first pair of locations indicate the alignment with respect to a first direction, and wherein the electrical measurements taken from the one or more current or voltage sources providing the currents to a second pair of locations indicate the alignment with respect to a second direction orthogonal to the first direction.
 21. The method of claim 20, wherein the one or more current or voltage sources comprise one or more current sources, and wherein determining the alignment comprises: comparing voltages used to provide the currents in the first pair to produce a first error indication with respect to the first direction, and comparing voltages used to provide the currents in the second pair to produce a second error indication with respect to the second direction.
 22. The method of claim 20, wherein the one or more current or voltage sources comprise one or more voltages sources, and wherein determining the alignment comprises: comparing magnitudes of the currents in the first pair to produce a first error indication with respect to the first direction, and comparing magnitudes of the currents in the second pair to produce a second error indication with respect to the second direction.
 23. The method of claim 15, wherein the first magnetic field is provided by a primary coil in the external charger.
 24. The method of claim 15, wherein the second magnetic fields are provided by coils at the different locations.
 25. The method of claim 15, wherein the first magnetic field and the plurality of second magnetic fields have different frequencies.
 26. The method of claim 15, wherein second central axes of the plurality of second magnetic fields are equidistant from a first central axis of the first magnetic field at the different locations.
 27. The method of claim 15, wherein second central axes of the plurality of second magnetic fields and a first central axis of the first magnetic field are parallel.
 28. The method of claim 15, wherein at least two of the plurality of currents are provided by different current or voltage sources.
 29. The method of claim 15, wherein at least two of the plurality of currents are provided by the same current or voltage source.
 30. The method of claim 15, wherein the electrical measurements comprises voltages used to produce the currents.
 31. The method of claim 15, wherein the electrical measurements comprise magnitudes of the plurality of currents.
 32. The method of claim 15, wherein the electrical measurements are taken simultaneously.
 33. The method of claim 15, wherein the power provided to the implantable medical device is temporarily interrupted to take the electrical measurements. 